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(FILES) -- A file photo taken on January 13, 2015 shows a hippopotamus, also known as the common hippo, swimming in its enclosure at the San Diego Zoo, California. A great-great grandfather of the hippopotamus likely swam from Asia to Africa some 35 million years ago, long before the arrival of the lion, rhino, zebra and giraffe, researchers said on February 24, 2015. Analysis of a previously unknown, long-extinct relative also confirmed that cetaceans -- the group of whales, dolphins and porpoises -- are in fact the hippo's closest living cousins. AFP PHOTO/MARK RALSTONMARK RALSTON/AFP/Getty Images